1. Field of the Invention
This invention relates generally to methods and compositions for preventing corrosion, and in particular, to methods and compositions for inhibiting corrosion on the surface of steel and other ferrous or metal materials. Most specifically, the present invention relates to a low VOC, water-borne, zinc-rich corrosion inhibitire coating for a wide variety of ferrous materials, and other metal materials more noble than zinc.
2. Description of the Prior Art
Uncoated steel, and other ferrous or metal materials will begin to rust and corrode upon exposure to the atmosphere. The iron in steel and other ferrous materials electrochemically interacts with atmospheric oxygen to form a reddish corrosion product, ferric oxide (Fe.sub.2 O.sub.3). The reaction occurs most rapidly in moist air, indicating the catalytic activity of water. The formation of red rust is often times undesirable because it compromises the integrity of the iron product. The red rust can also undesirably stain or rub off on other materials with which the iron product may come into contact.
In order to prevent the problem of red rust formation, it has been known to coat the surface of the iron material with a zinc dust-containing coating. Once in electrical contact with the steel surface, the zinc metal then cathodically protects the steel by corroding sacrificially. This sacrificial corrosion is evidenced by the formation of desirable white rust.
Zinc is a fairly reactive metal, and in the presence of water, it can react to generate hydrogen. This outgassing consumes the zinc, lessening the protective capacity of the coating, and it can be a problem in and of itself. Until recently, most zinc-dust-loaded coatings for protecting steel were made utilizing organic solvents as carriers. Problems such as extensive outgassing, as well as rapid consumption of the active zinc did not occur because there was little or no moisture exposure in these hydrophobic environments to corrode the zinc. Hence, inhibitors of the zinc dust itself were usually not required.
Because of increasing concern over the detrimental effects of volatile organic emissions from metal finishing coatings, as well as the rest of the paint industry, water-borne coatings have substantially increased in popularity. However, the development of low VOC, water-borne, zinc-rich corrosion-inhibitive coatings has presented several problems which did not exist in the analogous solvent-borne systems. The addition of water to the zinc-containing coatings generates hydrogen gas which is highly undesirable in closed systems. Also, water-borne systems have shown inherently poorer capabilities to protect the steel against corrosion.
Currently, the most widely used inhibitors for permanently passivating most light, active metals in aqueous corrosive environments, and therefore eliminating the gassing problem, are the alkaline earth and zinc salts of hexavalent chromium. They vary mostly in their degree of water solubility (in the order of Mg&gt;Ca&gt;Sr.gtoreq.Zn) and to a much smaller extent in their pH (Sr&gt;Ca&gt;Mg&gt;). The chromate anion is the active species. For the most part, low solubility strontium and zinc chromates are what have been used in metal primer coatings. The use of hexavalent chromium is undesirable, however, because it electrochemically passivates the zinc metal, thus reducing the ability of the zinc to cathodically protect the steel. Furthermore, the use of hexavalent chromium is an environmentally unacceptable solution to this problem.
The chromate ion, which is an excellent corrosion inhibitor for many metals, has been one of the most widely used for almost a hundred years. It has been used extensively as a paint pigment in metal primer coatings. For the past ten years, however, it has also been recognized as toxic and carcinogenic, and because of its health risks, has become highly regulated. With pressure for elimination being exerted by government regulations, continued use of chromium is incurring ever increasing economic penalties. Hence, there is a need for non-toxic substitutes, both from an economic and an environmental standpoint.
The present invention overcomes all of the above problems of the various prior art approaches by minimizing the problem of hydrogen outgassing, while simultaneously improving the corrosion-inhibition capabilities of non-chromate-containing, water-borne, zinc-rich coatings.
The present invention provides for water-borne, zinc-dust-loaded coatings which can be used for the protection of steel and other metals more noble than zinc. These coatings will perform this function in an environmentally friendly manner that will comply with all the latest regulations for low VOC and freedom from heavy toxic metals. These and other advantages of the present invention will be readily apparent from the description, discussion, and examples which follow.